This invention relates to a reaction force transfer method to reduce machine base motions due to stage acceleration in high through-put motion applications.
A large class of precision machines position and move a work piece relative to a tool. The work piece, the tool, or both are moved along a number of motion axes. Because of the high precision required, such machines are mounted to provide substantial isolation from ambient vibration sources. Modem passive isolation systems are extremely effective at attenuating the amplitude of ambient environmental vibrations that enter the machine. Isolation is typically accomplished by vertically supporting a large machine mass such as a granite base on a soft spring, yielding a very low system natural frequency. Damping is added to minimize disturbances near this low natural frequency.
A major disturbance arises when a horizontal axis of a machine accelerates or decelerates rapidly. The large force needed to accelerate a payload mass along an axis generates an equal reaction force into the machine's “stationary” structure according to Newton's Third Law. This reaction force accelerates the entire machine mass, including the machine base, which is, as stated above, typically a massive granite slab. The only resistance to the machine's lateral motion is the relatively soft horizontal spring rate of the isolation system. Compounding the problem is the low damping ratio of the isolators in the horizontal directions. The machine thus oscillates side-to-side with unacceptable amplitude and settling time.
This oscillation of the machine base may cause other problems. The machine base motion is a transient excitation to all other structures attached to it. This input will excite resonant frequencies in such structures. These structures may include vibration sensitive devices such as motion axes, metrology instruments, and the machine's tool (e.g., an atomic force microscope or optical system).
Active isolation systems are available that can counteract this machine oscillation problem. However, active isolation is extremely expensive. Alternatively, it is known that force cancellation methods can be used. In this case, a dummy axis is added to the machine and is programmed to make opposing moves which cancel the reaction forces caused by the real machine axes. In some machine configurations, this force cancellation technique can be elegantly implemented. In many other machine configurations, however, this solution is impractical because of the doubling of power requirements, increased space requirements, and increased expense.